As mobile devices have been increasingly developed and the demand for such mobile devices has increased, the demand for secondary batteries has sharply increased as an energy source for the mobile devices. Among such secondary batteries is a lithium secondary battery having high energy density and discharge voltage, into which much research has been carried out and which is now commercialized and widely used.
A secondary battery has attracted considerable attention as an energy source for power-driven devices, such as an electric bicycle (E-bike), an electric vehicle (EV), and a hybrid electric vehicle (HEV), as well as an energy source for mobile wireless electronic devices, such as a mobile phone, a digital camera, a personal digital assistant (PDA), and a laptop computer.
A small-sized battery pack, in which a battery cell is mounted, is used for small-sized devices, such as a mobile phone and a digital camera. On the other hand, a middle or large-sized battery pack, in which a battery pack including two or more battery cells (hereinafter, also referred to as a “multi-cell”) connected to each other in parallel and/or in series is mounted, is used for middle or large-sized devices, such as a laptop computer and an electric vehicle.
A lithium secondary battery exhibits excellent electrical properties as described above; however, the lithium secondary battery has low safety. For example, when abnormal operations, such as overcharge, overdischarge, exposure to high temperature, and an electrical short circuit, of the lithium secondary battery occur, decomposition of active materials and an electrolyte, which are components of the battery, is caused with the result that heat and gas are generated and high-temperature and high-pressure conditions caused by generation of the heat and the gas accelerate the above-mentioned decomposition. Eventually, a fire or explosion of the lithium secondary battery may occur.
For this reason, the lithium secondary battery is provided with a safety system, such as a protection circuit to interrupt electric current when the battery is overcharged or overdischarged or when overcurrent flows in the battery, a positive temperature coefficient (PTC) element whose resistance greatly increases so as to interrupt electric current when the temperature of the battery increases, and a safety vent to interrupt electric current or to exhaust gas when pressure increases due to generation of the gas. In case of a small-sized cylindrical secondary battery, for example, the PTC element and the safety vent are usually disposed at the top of an electrode assembly (a generating element) having a cathode/separator/anode structure, which is mounted in a cylindrical container. In case of a small-sized prismatic or pouch-shaped secondary battery, on the other hand, the protection circuit module and the PTC element are usually mounted at the upper end of a prismatic container or a pouch-shaped case, in which the generating element is mounted in a sealed state.
The safety-related problem of the lithium secondary battery is even more serious for a middle or large-sized battery pack having a multi-cell structure. Since a plurality of battery cells is used in the multi-cell battery pack, abnormal operation of some of the battery cells may cause abnormal operation of the other battery cells with the result that a fire or explosion of the battery pack may occur, which may lead to a large-scale accident. For this reason, the middle or large-sized battery pack is provided with a safety system, such as a fuse, a bimetal, and a battery management system (BMS), to protect the battery cells from overdischarge, overcharge, and overcurrent.
Meanwhile, as the lithium secondary battery is continuously used, i.e. as the lithium secondary battery is repeatedly charged and discharged, the generating element, electrical connection members, etc. are gradually degraded. For example, degradation of the generating element leads to decomposition of electrode materials and the electrolyte, by which gas is generated. As a result, the battery cell (the container or the pouch-shaped case) gradually swells. In a normal state of the lithium secondary battery, the BMS, which is the safety system, detects overdischarge, overcharge, or overcurrent of the battery pack to control/protect the battery pack. In a case in which the BMS does not operate in an abnormal state of the lithium secondary battery, however, the risk of the battery pack increases and thus it is difficult to control the battery pack in order to secure the safety of the battery pack. Since a middle or large-sized battery pack is configured to have a structure in which a plurality of battery cells is mounted in a specific case in a fixed state, the respective battery cells are further pressed in the limited case when the respective battery cells swell. In an abnormal state of the middle or large-sized battery pack, a lire or explosion may increasingly occur.
In connection with this case, FIG. 1 is a typical view showing circuitry of a conventional middle or large-sized battery pack.
Referring to FIG. 1, a conventional middle or large-sized battery pack 1 includes a battery module 10 constituted by a plurality of battery cells, a BMS 60 to detect information regarding an operation state of the battery module 10 and to control the battery module 10 based on the detected information, and a power connection and disconnection part (relay) 70 to perform connection and disconnection between the battery module 10 and an external input and output circuit (inverter) 80 according to an operation command of the BMS 60.
In a case in which the battery module 10 normally operates, the BMS 60 keeps the power connection and disconnection part 70 in an ON state. In a case in which abnormality of the battery module 10 is sensed, the BMS 60 switches the state of the power connection and disconnection part 70 to an OFF state to interrupt charge and discharge of the battery module 10. On the other hand, in a case in which the BMS 60 abnormally operates or does not operate at all, the BMS 60 does not perform any control. Consequently, the power connection and disconnection part 70 is kept in the ON state. As a result, charge and discharge of the battery module 10 are continuously performed even in such an abnormal state.
In addition, the conventional middle or large-sized battery pack 1 includes a cell cover in which the battery cells are mounted and other sheathing members. Since the cell cover and the other sheathing members are made of a pressed metal material, however, a short circuit may easily occur due to contact therebetween. As a result, insulativity of the battery pack is not fundamentally secured. Particularly, in a case in which external force is repeatedly applied to the battery pack, such a problem may become more serious.
Therefore, there is a high necessity for technology that is capable of improving insulativity of a middle or large-sized battery pack while solving the above problems, thereby fundamentally securing safety of the battery pack.